The majority of present day integrated circuits (ICs) are implemented by using a plurality of interconnected field effect transistors (FETs), also called metal oxide semiconductor field effect transistors (MOSFETs or MOS transistors). The ICs are usually formed using both P-channel and N-channel FETs and the IC is then referred to as a complementary MOS or CMOS circuit. Certain improvements in the performance of FET ICs can be realized by forming the FETs in a thin layer of semiconductor material overlying an insulator layer. Such semiconductor on insulator (SOI) FETs, for example, exhibit lower junction capacitance and hence can operate at higher speeds. It is advantageous in certain applications, however, to fabricate at least some devices in the semiconductor substrate that supports the insulator layer. The devices formed in the substrate, for example, may have better thermal properties and can support higher voltages than devices formed in the thin semiconductor layer. Such devices can be used, for example, for band gap references and for thermal sensing.
As the complexity of the integrated circuits increases, more and more MOS transistors are needed to implement the integrated circuit function. As more and more transistors are designed into the IC, it becomes important to shrink the size of individual MOS transistors so that the size of the IC remains reasonable and the IC can be reliably manufactured. Shrinking the size of an MOS transistor implies that the minimum feature size, that is, the minimum width of a line or the minimum spacing between lines, is reduced. MOS transistors have now been aggressively reduced to the point at which the gate electrode of the transistor is less than or equal to 45 nanometers (nm) in width. Methods previously used to fabricate devices in the substrate of an SOI structure, however, have not be able to achieve the same minimum feature size in substrate devices as are realized in the devices formed in the thin semiconductor layer.
Accordingly, it is desirable to provide a method for fabricating SOI devices having small feature sizes. In addition, it is desirable to provide a self aligned method for fabricating small feature size substrate devices in an SOI device. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.